Department of Transport: A Profile of Sandra Encalada

Picture
a tubular liquid world strewn with zippers and zeppelins…

The zippers are railways of a sort—like cog railways. The
zeppelins, vast lumpy sacs stuffed not with gas but with freight, whoosh along the
zippers at mind-bending speeds, pulled by long robot legs that walk in a
frenetic, cog-by-cog tiptoe.

This surreal vision is brought to you in part by
cell biologist Sandra Encalada, an Arlene and Arnold Goldstein Assistant
Professor of Molecular and Experimental Medicine and one of the newest faculty
members on The Scripps Research Institute’s California campus. Encalada is an
expert on the liquid world within the axon—a nerve cell’s (neuron’s) output
stalk—and specifically on its cargo-transport system, whose smooth operation is
essential for neuronal health. “An axon is thin and often very, very long, and
if its tracks for moving neurotransmitters, synaptic proteins, and other key
molecules are blocked, the neuron may die,” she says.

While finishing her postdoctoral work at the University
of California, San Diego (UCSD) last year, Encalada was first author of an
important paper in the journal Cell that
significantly clarified the picture of axonal transport. Encalada and her
colleagues attached fluorescent reporter-molecules to cargo proteins in cultured
mouse neurons, then watched as the sac-like vesicles containing this glowing
freight moved to and fro along axonal railways (called microtubules). Using
genetic and other techniques, she systematically removed subunits of the
vesicle-hauling, microtubule-gripping “motors” (known as kinesins and dyneins) to
see whether the dynamics of transport changed—and thus which subunits had been
in use.

Her data unequivocally validated a model of axonal
transport in which groups of kinesins and dyneins remain stably attached to an
individual vesicle. “We think that these motors are activated and deactivated
depending on the direction in which the vesicle will travel, but they remain
bound to their vesicles throughout their journey, and it is likely that motors
disengage the microtubule when the vesicle stops, and when reactivated the
motors re-engage the microtubule to move the vesicle again,” Encalada says.

Kinesins haul vesicles from the main body of a neuron
toward the axon terminus; while dyneins do the reverse. Encalada’s study found
evidence that some vesicles have both kinesins and dyneins, and thus can run
either way. Considering the simplicity and fragility of these nano-machines,
they move their cargo with astonishing speed: Encalada clocked vesicles going
as fast as 2.8 microns per second—the human-scale equivalent of several hundred
miles per hour.

A Less Linear Life Path

Encalada’s own path through life has been less linear,
more serendipitous. She was born in Quito, Ecuador, and didn’t plan on becoming
a lab-based biologist until she was in her mid-20s. She spent time in Palo
Alto, California, as a child, when her father was in graduate school at
Stanford University, then moved to the United States as a teenager to attend
the Armand Hammer United World College of the American West —a two-year international
prep school in New Mexico—with a full scholarship.

Later she attended a small college in Indiana, majoring
in physics. “I was very quantitative,” she says. Her father, a journalist and
founder of an environmental group, had encouraged her to do well in math. “He
emphasized that good math skills would open up a lot of opportunities,” she says.

An undergraduate ecology project she did in
Puerto Rico sparked an interest in biological systems, and, after getting her
physics degree, she spent two years at the University of Florida, studying sea
turtle population genetics. She found that she liked lab work, and for her PhD
at the University of Oregon she switched to molecular genetics—and, among other
things, began studying kinesins.

Function and Dysfunction

At UCSD, she worked in the laboratory of cell biologist
and frequent Scripps Research collaborator Lawrence S. B. Goldstein. Like
others in the Goldstein lab, Encalada tried to understand not only how the
elements of axonal transport function normally, but also how their dysfunction
relates to the neurodegeneration seen in Alzheimer’s and Hungtinton’s diseases
and related conditions. However, in the Goldstein lab, Encalada started a new
avenue of research and focused on trying to understand how axonal transport
dysfunction might be involved in the initiation of prion diseases—disorders
where aggregates beget more aggregates and neuronal degeneration in either an
infective, genetic, or sporadic paradigm.

The potential clinical implications of her research were brought
to the attention of Arnold and Arlene Goldstein (no relation to UCSD’s Lawrence
Goldstein), whose philanthropy supports the study of such diseases. The couple had
funded an assistant professorship in the same Scripps Research department in
2009. Last year, their generosity and vision made it possible to create
Encalada’s position at Scripps Research.

In last year’s Cell
study, the vesicle-hauled freight protein used by Encalada and her colleagues
was one with real disease significance: the mammalian prion protein, PrPc,
which is linked to Creutzfeldt-Jakob disease, “mad cow,” and other prion
diseases. The normal function of this synapse-associated protein remains a mystery—subtracting
its gene from mice causes no clear problem. But scientists know that, in rare
cases, single copies (monomers) of PrPSc, a misfolded form of PrPc,
start aggregating into small clusters or “oligomers” that are somehow highly
toxic to neurons.

Worse, these toxic oligomers act as DNA-like templates
for assembling new copies of themselves, so that they can, in principle, pull
every free-floating monomer of PrPc in their vicinity into the PrPSc
pathogenic oligomer form—and thus can burn infectiously wherever PrPc
is abundant. They burn quickly, too: Prion diseases such as Creutzfeldt-Jakob
and “mad cow” disease can destroy the brain in a matter of months. Amyloid beta
and tau proteins in Alzheimer’s disease, alpha-synuclein protein in Parkinson’s
disease, and huntingtin protein in Huntington’s disease are all now suspected
of driving their diseases in a similar manner, but more slowly.

A striking feature of all these diseases is a
swelling of axons in affected neurons, as if blockages of normal transport
routes have led to buildups of undelivered molecules. “One hypothesis we’re
considering is that the pathogenic aggregates in these cases are killing
neurons by blocking axonal transport,” Encalada says. And since axonal
transport normally can be used to eject unwanted protein aggregates from
neurons, disruptions of axonal transport could create a vicious spiral of
worsening aggregate accumulation.

“In some of these diseases, the disruption of axonal
transport might even be an initiating event,” Encalada notes. “There’s already some
evidence for that, and it’s something that we want to study in our cell and
animal models here at Scripps in the context of numerous aggregation diseases
including ALS and the transthyretin amyloid diseases.”

In one ongoing project, Encalada and her
students are tracking pathogenic prion aggregates as they move along axons,
apparently carried at normal speeds by vesicles. She wants to know whether
these aggregates use these journeys in vesicles to corrupt fellow-traveling PrPc,
ultimately blocking axonal railways. That could end up being the first precise explanation
for how prions and other disease-linked protein aggregates terminate their
neuronal victims.

Encalada also wants to find out more clearly
how kinesins and dyneins are regulated, and whether their efficiency declines
significantly as organisms get older—which, if so, could help explain why most
neurodegenerative diseases are strongly linked to advancing age. “We’re now
planning a comprehensive comparison of axonal transport in early, mid, and late
life, using our animal models,” Encalada says.

These animal models include the nematode worm C. elegans, which is favored by many
labs for its relatively quick and easy genetic manipulability. Although it is eons
removed from a brainy primate like Homo
sapiens, the worm makes a surprisingly good model for some aggregate-linked
diseases. Among these are the transthyretin (TTR) amyloid diseases, which
originate from a liver-produced protein. Scripps Research Professor Jeffery
Kelly, who chairs the Department of Molecular and Experimental Medicine, recently
devised a small-molecule drug, tafamidis (now approved for use in Europe and
under review by the U.S. Food and Drug Administration), that inhibits the
formation of toxic TTR aggregates—but still no one really knows why these aggregates
are toxic. “We’re trying to find out whether they, too, are involved in
disrupting axonal transport,” Encalada says.

The work is keeping her busy. An avid runner
and swimmer, Encalada is a member of the Triathlon Club of San Diego, but says
she has had to cut down on races while she sets up her lab: “The difference
between being a postdoc and a faculty member—oh boy!”

Send comments to: press@scripps.edu

Sandra
Encalada’s work relates to conditions including Alzheimer’s disease,
Huntington’s disease, ALS, and prion diseases such as “mad cow.”